5. Liquefaction Unit (L-4000)
5.1 Design Concept and Basis
Offshore liquefaction facilities have different design criteria and process requirement from the on-shore liquefaction facilities. Thermodynamic efficiency is the most important factor for the large scale LNG production plant in both on-shore and offshore processes, but the offshore liquefaction facilities have different design requirements compromising and/or competing with the thermodynamic efficiency, such as compactness, safety, stability on the motion and design code and practice for the marine environments.
The process design of this project aims to produce 3.5 MPTA LNG with a general liquefaction theory and process which have been being used worldwide as the main purpose of this project is to develop the LNG FPSO MPDS (Muti-Purpose Dynamic Simulator) to be mainly used for the operator training of LNG FPSO topside process.
To achieve the purpose above, the liquefaction process is designed based on the pre-cooled mixed refrigerant cycle because this is the most popular LNG refrigerant cycle and still being considered first alternative for the floating LNG liquefaction process based on the energy efficiency, performance and availability which has been proven worldwide. The pre-cooling cycle uses propane refrigerant and consists of four (4) propane chillers and compression system. The Main Cryogenic Heat Exchanger uses Mixed Refrigerant to liquefy the pre-cooled natural gas and the MR system consists of Scrub column to remove heavy hydrocarbon, MCHE, MR compression system and End flash scrubber and compressors.
The process consists of the liquefaction, refrigeration and the end flash compression of the LNG product. The Liquefaction Unit (U-4000) mainly includes the following process equipments:
- Propane Chillers (U-E-4015/4016/4017/4018)
- Scrub Column System (L-C-4001)
- Main Cryogenic Heat Exchanger (L-E-4007)
- Refrigeration System
5.2 Process Description
5.2.1 NGL Extraction and Liquefaction System
The purpose of the Liquefaction Unit, L-4000, is to produce liquefied natural gas (LNG). The Propane Pre-cooled Mixed Refrigerant process cycle is used for this purpose.
Treated natural gas exiting Unit A-1000 is cooled against high-high pressure (HHP) propane chiller in L-E‑4015 upstream of the Dehydration Unit, D-2000, to reduce the water load on the molecular sieve beds. Dry, sweet natural gas from downstream of the Mercury Removal Unit, M-3000, is then cooled and partially condensed against high pressure (HP) and medium pressure (MP) propane chillers in L-E‑4016 and L-E‑4017. The temperature of the natural gas stream to the scrub column (L-C-4001) is controlled at -13.5°C via a bypass valve (L-FCV-4001) over L-E-4016 and L-E-4017.
The recombined and temperature controlled natural gas stream (-13.5°C) enters the scrub column L-C-4001 where heavy components and aromatics are removed to avoid freezing in the Main Cryogenic Heat Exchanger (MCHE, L-E‑4007), to meet the C5+ specification of 0.1 mol % for the LNG delivered to the customer and to recover adequate amounts of ethane and propane for the refrigerant make-up and export.
Condenser duty for the scrub column is provided by both low pressure (LP) propane L-E-4018 (which cools the scrubber column overhead to –37.5°C) and the warm bundle of the MCHE, which partially condense the vapours from the overhead of the column by controlling the temperature (-45°C) through bypass temperature valve (L-TCV-4007). This condensed liquid is separated in the reflux drum L-V-4001 and pumped back (L-P-4001 A/B) into the top of the scrubber column (L-C-4001).
The Main Cryogenic Heat Exchanger (MCHE, L-E-4007) is a large, vertical, multi-tube, spiral wound heat exchanger. The longitudinal distance between the tubes in a layer and the tube inclination are kept constant for all layers. The tubes are connected to tube sheets at each end of the heat exchanger and each layer contains tubes from all the different streams.
The MCHE (L-E-4007) can consist of three tube bundles, warm bundle (lower part), middle bundle (middle part) and cold bundle (upper part). The tube bundles are connected together in one shell. The exchanger is operated in counter current mode, with the evaporating refrigerant flowing downwards on the shell side and the high pressure, condensing fluid (refrigerant and natural gas in separated tubes) flowing upwards on the tube side. Each tube bundle has its own reservoir with liquid distributor at the top of the bundle to ensure an even distribution of shell-side refrigerant from the top to the bottom of a tube bundle.
The vapour from the reflux drum is routed to middle bundle of the MCHE. Here it is condensed and sub-cooled. This stream is then further sub-cooled in the cold bundle. Part of the liquid from L-V-4001 is fed forward through L-LCV-4002 to the MCHE in order to reduce the load of the scrub column and the fractionation facilities within the upper limit of sales LNG quality specifications.
The pressure of the LNG stream leaving the MCHE (L-E-4007) cold bundle is reduced through a liquid expander (L-KG-4002). Overhead vapour of the deethaniser column (F-C-5001)) of the fractionation unit is routed to a dedicated low-pressure circuit of the MCHE installed over the three tube bundle, warm, middle and cold bundles. The liquid natural gas from L-KG-4002 and the liquefied deethaniser overheads are mixed and routed to upstream of end flash vessel L-V-4003.
The end flash gas from the end flash vessel (L-V-4003) flows through the Light MR Cold Recovery Exchanger (L-E-4008) to recover the cold energy and then is mixed with boil off gas from LNG storage and loading and routed to the HP fuel gas system (U-6000) via 3-stage end flash compressor (L-K-4001 A/B/C). The LNG separated in the end flash vessel (L-V-4003) is pumped (L-P-4002 A/B) to storage at a temperature of about -160 0C.
Natural gas liquid (NGL) from the Scrub Column (L-C-4001) is sent to the Deethanizer (F-C-5001) in Fractionation Unit (F-5000) through L-FCV-4004 under the level control (LICA-4002) of the scrubber column bottom for further fractionation processing to recover the natural gas liquid into ethane, propane, butane and condensate.
Figure 5,6,7 shows the overall process of the scrubber column and liquefaction systems.
Figure 5 Overall Process of the Scrubber Column Unit
Figure 6 Overall Process of the Main Cryogenic Heat Exchange Unit
Figure 7 Overall Process of the LNG Expander and End Flash Compressor Unit
5.2.2 Refrigeration Systems
The refrigeration systems consist of two major systems, Propane Refrigeration System and MR Refrigeration System.
220.127.116.11 Propane Refrigeration System
The propane refrigeration system provides the propane refrigerant to the following propane chillers to cool down several process streams before and after several process units which require a cold energy source. Each propane chiller is designed as multi-stream and spiral tube exchanger. Each chiller is operated in counter current mode, with the evaporating propane refrigerant flowing downwards on the shell side and the high pressure, condensing fluid (refrigerant and natural gas in separated tubes) flowing upwards on the tube side. Each chiller is operated in different pressure level of the shell side which provides different operating temperature.
[table id=38 /]
Propane vapours from the shell sides of propane chillers are routed via knock-out drums to the propane compressor (L-K-4004 A/B/C/D). L-K-4004 is a 4 stage centrifugal compressor driven by a gas turbine (L-KT-4004) with a variable speed electric starter-helper motor / generator (L-KM-4004) on a common shaft.
The starter-helper motor / generator provides:
- starter power, to bring the compressor / turbine combination to running speed.
- electricity from the excess gas turbine power, if available.
- additional shaft power during normal operation.
Desuperheating, condensation and subcooling of the propane are achieved by using air coolers, L-E-4019 and L-E-4020, respectively. The condensed propane is collected in the propane accumulator L-V-4009.
The propane liquid from the accumulator (L-V-4009) is sub-cooled in each propane chiller against returning propane refrigerant on the shell side, pressure let-down across the JT valve and evaporated in the shell side of the propane chiller, cooling the MR (L-E-4015/4016/4017/4018), sweat natural gas (L-E-4015), sweat and dry natural gas (L-E-4016/4017), scrubber column overhead (L-E-4018) and deethanizer overhead (L-E-4033) and the tubeside propane refrigerant.
The propane flow rate through the JT valve for each propane chiller is controlled by temperature controller of MR at the exit of each propane chiller.
Figure 8 shows the overall process of the propane refrigeration system.
Figure 8 Overall Process of the Propane Refrigeration System
18.104.22.168 Mixed refrigerant system
The cooling duty for the liquefaction of the natural gas is provided by a mixed refrigerant (MR) cooling cycle (mixture of nitrogen, methane, ethane and propane).
The following is the MR composition which is used to this process design.
[table id=39 /]
MR vapour from the MCHE shell side is routed to the MR suction drum (L-V-4004) and then compressed in three (3) stage centrifugal compressor (L-L-4002 A/B/C).
Each stage has an air after-cooler, L-E-4012, L-E-4013 and L-E-4014, respectively.
After 4 levels of propane chilling the MR vapour / liquid stream is separated in the HP MR separator (L-V-4002). The light mixed refrigerant (LMR) vapour is condensed and sub-cooled in the MCHE against returning refrigerant on the shell side, pressure let-down across the cold JT valve (L-FCV-4005) and evaporated in the shell side of the MCHE, cooling the natural gas and the tube side refrigerant.
A part of LMR is routed to the Recovery Exchanger (L-E-4008) and condensed/sub-cooled against the cold vapour from the end flash vessel (L-V-4003). The LMR flow through the L-E-4008 is controlled by the temperature of the end flash vapour downstream of L-E-4008.
The liquid heavy mixed refrigerant (HMR) is subcooled in the warm middle bundle. The liquid leaves the cold end of the middle warm bundle and is routed to the HMR expander (L-G-4001). (If the expander is not running, the HMR is flashed across the warm JT valve (L-FCV-4006)). The HMR is then mixed inside the MCHE with the low pressure LMR from the cold bundle. The combined MR stream is vaporised and superheated by the warm bundle before being re-compressed in the MR compressor
Figure 9 shows the overall process of the MR refrigeration system.
Figure 9 Overall Process of the MR Refrigeration System
LNG FPSO MPDS (Multi-Purpose Dynamic Simulator) PROJECT SERIES